Testing corrosion inhibitors



3,110,567 Patented Nov. 12, 1963 of Delaware No Drawing. Filed Nov. 8, 1957, Ser. No. 695,233

4 Claims. (Cl. 23-239) The present invention relates to a method for testing the effectiveness of an adsorption type insulating organic corrosion inhibitor for inhibiting corrosion of metal surfaces.

In the prevention of corrosion of ferrous metals and, particularly, in the petroleum industry and, more specifically, in the producing phase of the petroleum industry, the metals are subjected to contact not only with oil and brine but also with naturally occurring gases. It appears that regardless of what other characteristics a corrosion inhibitor must have, presumably it must adsorb at an interface and, more specifically, at a water-metal interface, oil metal interface, or the like.

In the past few years the efforts of the corrosion engineer have been taxed by the large number of products being marketed as corrosion inhibitors by various manufacturers. These products are submitted to him together with a certain amount of data to support their use as inhibitors. The data are usually authentic and reliable yet all too often when these products are applied to an actual corrosive system the results are disappointing due to the incompatibility of the inhibitor and the corrosive system.

As a practical matter, therefore, the corrosion engineer must select a comparatively limited number of inhibitors for field trial from the large group'available. The selection is usually made on the basis of a standardized laboratory test such as that adopted by the National Association of Corrosion Engineers or by field experience. Testing of this type on large numbers of compounds is discouraging, however, particularly in view of the specificity of environment.

The present invention provides a method which will allow the corrosion engineer to select the best inhibitor for a specific environment under field conditions. The method of the invention is rapid and inexpensive and eliminates the need for expensive and delicate instruments.

The principle underlying the present invention is the fact that metals in the electromotive force series of elements tend to displace the metal ions from a solution of a water-soluble, ionizable salt of a metal lower in the series. Thus, for example, if an active iron surface is exposed to a solution of a copper salt, free copper will plate spontaneously on the iron surface as it is displaced from the salt solution by iron ions. When the iron surface is altered by coating with an adsorbent insulating type organic corrosion inhibitor, however, the cell potential will be altered as will the amount of copper plated spontaneously from solution. It has been found that this alteration of the cell potential can be used as a measure of the protection expected from an inhibitor in the system. Thus according to the method of the present invention an expendable test sample of the metal to be inhibited is first exposed within a corrosive fluid containing an adsorption type insulating organic corrosion inhibitor and then exposed to a solution of a water-soluble, ionizable salt of a metal lower in the electromotive force series of elements than the metal to be inhibited. The amount of the metal lower in the electromotive force series of elements which has been deposited on the expendable ferrous test sample is then determined and the amount of deposited metal indicates the extent of the insulating film and hence the effectiveness of the corrosion inhibitor.

For the reason that corrosion of ferrous metal surfaces is one of the most important corrosion inhibiting probnated as iron or a ferrous metal.

There are several metals below iron in the electromotive series of elements which can be used for deposition on the ferrous test sample but for practical and other reasons, the use of copper, particularly in the form of its sulfate salt, appears to be most advantageous. Copper sulfate is a reagent that is readily available at low cost. The high cell potential available in an iron-copper system provides a substantial driving energy for plating out of the copper and the color difference between iron and copper permits visual determination of the extent of plating out. In addition, the amount of copper plated out can be determined colorimetrically by dissolving it in 5% ammonium hydroxide solution containing 2% potassium persulfate. The blue color of the complex diammonium copper hydroxide ion so developed can be compared with standards prepared by dissolving known quantities of copper powder in the same reagent. For the above reasons and for ease of presentation, the method of this inevntion will be further described with respect to copper as the metal lower than iron in the electromotive series of elements.

Although copper sulfate is the preferred copper salt, other inorganic copper salts such as copper chloride and copper nitrate can be used. Certain organic copper salts which are stable under conditions for use such as those of sulfated alcohols and monocarboxy and dicarboxy acids can be used as long as they are soluble and ionizable. For example, copper acetate and the copper salts of sulfated butanol, sulfated pentanol and diglycollic acid are suitable. Also, the copper ion can be furnished by what is commonly referred to as a complex or coordination compound such as the one obtained by the reaction of ammonia and a copper ion. Essentially all that is necessary is that the copper salt be water-soluble and ionizable so as to provide copper ions in solution for displacement. Salts of other metals below copper in the electromotive series of elements such as those of gold, silver and platinum can be used but for the reasons outlined above, copper is preferred.

In routine screening it is seldom desirable to determine the amount of copper plated. It is usually more important to determine the concentration of inhibitor required to prevent the plating of copper. Accordingly, a suitable procedure that has been devised and which was employed in the subsequently described tests includes first allowing one inch square, sand-blasted coupons prepared from 0.006 inch thick shim stock to contact brine from the corrosive source for 5 minutes. By treatment of the water wet metal, the factor of oil wetting is adequately considered. The coupons are transferred without drying to solutions containing various known concentrations of the inhibitor inthe oil from the corrosive source in which they are allowed to remain for an additional 5 minutes. The coupons are then transferred to a 10% solution of copper sulfate where they are allowed to remain for 30 I seconds. Blank determinations using uninhibited oil from the corrosive source are made in the same manner. The lowest concentration of inhibitor required to prevent-the plating of copper is taken as the criterion for a pass. In most instances this can be determined visually.

The above procedure will preclude any wetting studies on the spot. It maybe desirable to study wetting along with this method and this can be done by drying the coupon after exposure to the brine and prior to treatment with the inhibited oil.

The method of this invention is primarily designed to select from a group of known corrosion inhibitors although in some instances it can be employed to evaluate new materials as inhibitors. The method pie-supposes that a film forming corrosion inhibitor is available. For example, no consideration should be given to a chemical having a limited film life such as a primary amine. Such a material under some circumstances gives apparently good results in this test, but it will have poor'film life and will actually be a poor inhibitor. Similarly a compound which acts strictly by cationic activity rather than through an adsorption mechanism, such as a quaternary ammonium salt, will show a poor result on the test, although in practice such materials may be good inhibitors.

It is evident that the method of this invention provides primarily a qualitative evalution. pects must, in some instances, 1b further evaluated since other factors such as time and temperature can be im- For example, where adsorption is thermally activated, the rate is exponentially dependent upon tern- Further, there may be a time factor in the adsorption process caused either by a comparatively sluggish adsorption rate or poor mobility of the adsorbing molecule on the metal surface prior to equilibrium. The method of this invention depends upon a rapid adsorption of an insulating type inhibitor and .in actual use the inhibitor has a much longer contact time and hence a portant.

perature.

longer time to reach the equilibrium state.

The above described procedure was employed for the evaluation of several comznercial corrosion inhibitors, which had been proved by field use, in freshly produced corrosive fluids at the well head from several producing oil wells. Records of actual corrosion experience in these producing oil wells were available for comparison and correlation. The data obtained are set forth in Table I The quantitative aspenetration from 3.44 to 0.72 MPY in 3 weeks.

ing properties :of new materials.

ing to the procedure described above.

below. better than the commercial inhibitor being used in this Table 1 Min. Pass Well Pool or Field Inhibitor Cone, Field Data p.p.m.

Sexton #9 gggi D p ov d e b mhlbltori C 600+ Sexton #12 Rh0dessa 288+ Same as above.

Inhibitor E was the best of the e i this Cotton Valley Greenwood. C 288i location. 5 n

Haynesville Haynesville }Inl1ibitor D was superior to inhibitor F.

Reynolds #1 Waterloo D 2, 000+ Despite the use of this material repeated rod parts have been encountered.

1,200+ 1, 200+ These chemicals have failed completely here. Stevens #1 Logausport 1,200 Pits increased from to penetration with I, O and H.

I G 0) These products were used with no success. Wilson #1 Iickens h 1 000 InhibltonD was then retried at increased cone. to give protection.

School Board #1 Gumvme }Gc1)d results have been obtained with C and Ohildress #1 Tinsley 600 @g/Z a g i D 600 }Tlieistri1 ivzililills'itzzglienbemg treated adequately oo in ,6 o 600 1, 600+ Inhibitor F has been found superior. In- Oherry Homes #16. J'acksonboro.-. 1 000 hibitor J failei 1 Will not; pass.

The criterion for passing the copper ion test is the concentration of the inhibitor in parts per million required to inhibit completely the plating out of copper.

In such a well the produced water is mixed with fresh water before injection and the optimum ratio of produced to fresh water is to be determined. Using a proved commercial inhibitor and water from the Haynesville, Louisiana, Operators Committee #22-4 injection well, it was determined that a 50-50 mixture of fresh water to produced Water was more corrosive than a 70-30 ratio. It required 600 p.p.m. of an inhibitor to pass while at a 7 030 ratio only 350 p.p.m. of inhibitor was required for a pass. Iron analysis at the injection well confirmed that the 5040 mixture was indeed more corrosive. Further the use of the inhibitor in the 7040 mixture reduced the The application of the method of this invention to fluids shipped to the laboratory may lead to errors in selection due to the changes which occur in well fluids with time. These changes are not fully understood, but they have been long recognized. Bearing this in mind, however, the method of this invention can be employed to determine preliminarily in the laboratory the corrosion inhibit- For example, samples of the fluids from the Hays Gas Unit No. l, Greggton, Texas, well were shipped to the laboratory for the evaluation. A new material was tested in these fluids accord The chemical structure of the material used in this test was significantly different from that of a commercial corrosion inhibitor actually being employed in this well. The new material was found to give no copper plate at p.p.m. while the commercial inhibitor required 300 p.p.m. to pass. A field trial showed the selected compound to be somewhat well with iron content of the produced fluids being reduced from an average of 124.2 p.p.m. to 97.9 p.p.m.

In another instance, samples of fluid from the Cotton Valley Operators Committee Grant #2 well at Cotton Valley, Louisiana, were shipped to the laboratory. A well known commercial inhibitor was being used with success in this well. A formulation was selected Which was com- 5 parable to the commercial material but which varied considerably in structure. Both the experimental and the commercial compounds passed the copper test at 200 p.p.m. indicating that they should be comparable in use. A sample of this material was then tried in the well. Table 2 shows the results of actual corrosion experience in the form of coupon losses and iron analyses.

Although the test was of short duration it is obvious that there is little change in the protection afforded the system. This was to be predicted since both materials looked the same in the copper ion displacement test.

The method of this invention has been found to correlate quite closely with static weight loss tests and with such standard tests as the ASTM Spindle Test. The static weight loss test which has been adopted by the National Association of Corrosion Engineers as a standard screening test has been correlated with the copper ion displacement test of this invention. Table 3 shows this correlation.

Table 3 Copper Ion Test, Percent Protection Inhibitor p.p.m. Required to NACE Test Inhibit Copper The copper ion displacement test of this invention also has been found to correlate quite well with the ASTM T urbine Oil Test (Federal Specifications VVL79le Method 4011.2). Table 4 shows the results of this correlation in which several compounds have been compared Still another rather close correlation has been found with a test for evaluating inhibitors for tanker corrosion. This test has been described by Malcolmson et al. in a paper presented at the November 1952 meeting of the Soc. of Naval Architects. It involves subjecting the coupon to a sea water-hydrocarbon system for one week followed by a sea water-air system for an additional week. Table 5 shows the results of a typical test series.

Table 5 Inhibitor Copper Ion Test, Cycle Test, Perp.p.m. to Pass cent Protection Another valuable use of the method of this invention in the laboratory is in defining the response of inhibitors to certain variables in the corrosion process. It has been found to be adaptable to a more critical evaluation of the factors contributing to the process of corrosion and to the inhibitor function. For example, such things as pH response, salinity efiects or temperature response can be rapidly determined. The results must be confirmed by weight loss tests, but fewer compounds will have to be handled in such tests.

Adsorption type insulating organic corrosion inhibitors can be cationic active or anionic active. The most widely used types of corrosion inhibitor are cationic active. These inhibitors generally are comparatively high molar organic compounds containing one or more basic nitrogen atoms, such as higher fatty acid amine, cyclic amidines, amino amides, comparable compounds derived from rosin, naphthenic 'acid, etc. Due to the much wider use and much greater commercial importance of the cationic class of materials it will be noted that previous examples have been limited to the use of cationic inhibitors although obviously if use were so indicated anionic inhibi tors could be tested. Anionic active inhibitors contain as anions a hydrophobic group, which can be essentially a large hydrocarbon radical, and as cations an alkali or alkaline earth metal, hydrogen, ammonium, amino etc. radical. Examples of this type of inhibitor are the sodium, potassium, calcium, ammonium, and trimethyl amino salts of mahogany acids and alkyl aryl sulfonic acids as well as the free acids themselves.

I cl-aimf 1. A method for testing the effectiveness of an adsorp-' tion type insulating organic corrosion inhibitor for inhibiting the chemical reactions of corrosion of metal surfaces which comprises exposing an expendable test sample of the metal to be inhibited within a corrosive fluid containing an adsorption type insulating organic corrosion inhibitor, exposing the expendable test sample to a solution of a water-soluble ionizable salt of a metal lower in the electromotive force series of elements than the metal to be inhibited, and determining the amount of the metal lower in the 'electromotive force series of elements which has been deposited on the expendable test sample.

2. A method for testing the effectiveness of an adsorp tion type insulating organic corrosion inhibitor for in hibiting the chemical reactions of corrosion of metal surfaces which comprises exposing an expendable test sample of the metal to be inhibited within a corrosive fluid containing an adsorption type insulating organic corrosion inhibitor, exposing the expendable test sample to a solution of a water-soluble ionizable salt of a strong inorganic acid and a metal lower in the electromotive force series of elements than the metal to be inhibited, and determining the amount of the metal lower in the electromotive force series of elements which has been deposited on the expendable test sample.

3. A method for testing the eflectiveness of an adsorption type insulating organic corrosion inhibitor for inhibiting the chemical reactions of corrosion of ferrous metal surfaces which comprises exposing an expendable ferrous test sample within a corrosive fluid containing an adsorption type insulating organic corrosion inhibitor, exposing the expendable ferrous test sample to a copper 7 sulfate solution, and determining the amount of copper which has been deposited on the expendable ferrous test sample.

4. A method for testing the effectiveness of an adsorption type insulating organic corrosion inhibitor for inhibiting the chemical reactions of corrosion of ferrous metal surfaces which comprises exposing an expendable ferrous test sample within a corrosive fluid containing an adsorption type insulating organic corrosion inhibitor, exposing the expendable ferrous test sample to a copper acetate solution, and determining the amount of copper which has been deposited on the expendable ferrous test sample. 7

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Swartz et a1.: Surface Active Agents, vol. 1, 1949,

K rotov: Doklady Akad. Nauk,'S.S.S.R., vol. 76, #4, pp. 559-562 (1951), February 1. 

1. A METHOD FOR TESTING THE EFFECTIVE OF AN ADSORPTION INSULATING ORGANIC CORROSION INHIBITOR FOR INHIBITING THE CHEMICAL REACTIONS OF CORROSION OF METAL SURFACES WHICH COMPRISES EXPOSING AN EXPENDABLE TEST SAMPLE OF THE METAL TO BE INHIBITED WITHIN A CORROSIVE FLUID CONTAINING AN ADSORPTION TYPE INSULATING ORGANIC CORROSION INHIBITOR, EXPOSING THE EXPENDABLE TEST SAMPLE TO A SOLUTION OF A WATER-SOLUBLE IONIZABLE SALT OF A METAL LOWER AN THE ELECTROMOTIVE FORCE SERIES OF ELEMENTS THAN THE METAL TO BE INHIBITED, AND DETERMINING THE AMOUNT OF THE METAL LOWER IN THE ELECTROMOTIVE FORCE SERIES OF ELEMENTS WHICH HAS BEEN DEPOSITED ON THE EXPENDABLE TEST SAMPLE. 